This invention relates to a recording medium made of Fe.sub.x N (x=2-3) suitable to perpendicular magnetic recording used for magnetic tapes, floppy disks, etc. and a fabrication method therefor.
In this specification substances including, Cr, whose concentration is not greater than 10% in number of atoms, besides Fe.sub.x N, are collectively called Fe.sub.x N.
Recording density in the field of the magnetic recording has been remarkably increased. In particular, the perpendicular magnetic recording method proposed by Iwasaki et al. of Tohoku University differs from the inplane recording method used in practice at present and it is characterized in that the self-demagnetizing effect becomes smaller with increasing recording density. Therefore, attention is paid thereto as a future high density magnetic recording method and researches are effected intensively thereon.
In order to realize this perpendicular magnetic recording, perpendicularly magnetized film having an easy magnetization axis perpendicular to the surface of the magnetic film is necessary as recording medium. Further, it is thought that a so-called double-layer structure, for which a high permeability magnetic film is disposed under this perpendicularly magnetized film, is more useful in practice than a so-called single layer structure consisting only of the perpendicularly magnetized film, because reproduction output for the former obtained by means of a magnetic head is twice as high as that obtained for the latter.
As the high permeability magnetic film are used well-known high permeability magnetic material for magnetic head, such as ferrite, Fe.sub.18 N.sub.2, Permalloy, Sendust, (Fe, Co, Ni)-(Si, B, C, P, Al-B) amorphous alloy, (Fe, CO, Ni,)-(Zr, Hf, Y, Ti, Nb, Ta, W, V, Mo, Cr) amorphous alloy, Fe.sub.x C, etc. applied on a carrier. The thickness of the film is usually 0.5-2.0 .mu.m.
As the perpendicularly magnetized film, are known alloy films of Co-Cr, Co-Cr-Rh, Co-V, Co-Ru, Co-O, Co, Co-Ni-Mn-P, etc. deposited by physical evaporation method (sputtering method, vacuum evaporation method, etc.), electroplating or chemical vapor deposition method (CVD method).
However, the main component of all of these perpendicularly magnetized films is Co, what is problematical with respect of cost and stable availability, because resources of Co are scarce. Further, for the magnetic substances whose main component is Co, there are risks of heat demagnetization and pressure demagnetization.
One of the methods for resolving these problems is to use perpendicularly magnetized films made of Fe.sub.x N, whose main component is Fe instead of Co. However, magnetic characteristics of the Fe.sub.x N films obtained by this method are somewhat inferior to those of Co-Cr films, which are the representative perpendicularly magnetized film.
The reason, why Fe.sub.x N (x=2-3) films form perpendicularly magnetized films, is assumed as follows. When a cross-section of an Fe.sub.x N film prepared by the physical evaporation method is observed by means of a scanning electron microscope, a columnar structure, in which crystalline grains are grown in the direction perpendicular to the surface of the film, is found. One of the reasons why Fe.sub.x N films form perpendicularly magnetized ones is the microscopic shape anisotropy due to these columnar crystallines (anisotropy due to the shape of individual columnar crystallines). Further the second reason therefor is that the condition represented by the following formula (1) is fulfilled, which means that the magnitude of this perpendicular anisotropy (Ku) is greater than the static magnetic energy 2.pi.Ms.sup.2 (Ms: saturation magnetization), when the magnetization is directed perpendicularly to the surface of the film. EQU Ku&gt;2.pi.Ms.sup.2 ( 1)
Usually, for Fe thin films, even if columnar crystallines of Fe are ideally aligned perpendicularly to the surface of the film, using the values of Ku and Ms for bulk, the values of the left and the right members of the formula (1) are about 9.times.10.sup.6 erg/cc and 1.8.times.10.sup.7 erg/cc, respectively, and therefore the condition represented by the formula (1) is not satisfied. Furthermore, since ideal columnar crystallines (length of longer axis/length of shorter axis is almost infinitely large) are never perfectly aligned in the direction perpendicular to the surface of the film, it is presumed that the value of the left member of the formula (1) is considerably smaller than 9.times.10.sup.6 erg/cc.
It is thought that the effect obtained by adding N is to accelerate the alignment of columnar crystallines perpendicular to the surface of the film and to decrease Ms to the extent that the formula (1) is valid. It is thought also that one of the reasons why Fe.sub.x N films form perpendicularly magnetized films is that N segregates on the grain boundary of the columnar crystallines.
In this case, when Fe.sub.x N has a hexagonal crystal structure and its C-axis is oriented perpendicularly to the surface of the film, since the crystal anisotropy of the hexagonal Fe.sub.x N is added to the microscopic shape anisotropy stated above, the perpendicular magnetic anisotropy (Ku, the left member of the formula (1)) becomes larger. Consequently, even if Ms is large, the Fe.sub.x N film forms perpendicular magnetized films. However, for the Fe.sub.x N alone, no alignment of the C-axis perpendicular to the film surface has been found.
As stated above, in an Fe.sub.x N film prepared by the physical evaporation method, columnar crystallines grow in the direction perpendicular to the surface of the film and the microscopic shape anisotropy of these columnar crystallines is a main reason for the perpendicular magnetic anisotropy of the Fe.sub.x N film. However, in the Fe.sub.x N film prepared directly on a non-magnetic substrate by a direct physical evaporation method, since the growth of columnar crystallines was insufficient, Ku of the Fe.sub.x N film was small and the saturation magnetic flux density (Bs=4.pi.Ms) of the Fe.sub.x N film for producing a perpendicular magnetized film was in a region of 0.20-0.65 Wb/m.sup.2 (2000-6500 G), which is somewhat smaller than Bs of a Co-Cr film, which is a representative perpendicular magnetized film.